This invention relates to a power converter composed of a plurality of rectifiers for producing a desired DC or AC output with an AC power supply as its input, or more in particular to a power converter system in which a higher harmonics current generated at the AC power supply side at the time of commutation of the rectifiers may be reduced.
A power converter system of this type is composed of a bridge connection of controlled rectifiers and a plurality of uncontrolled rectifiers or a bridge connection of only a plurality of controlled rectifiers and the firing phase of the controlled rectifiers is controlled for conversion from AC into DC power.
Such a power converter system utilizes the input AC power supply without using any special power supply for the commutation of the rectifiers. The quantity of electricity at the load side of the power converter, such as the rectified voltage or the rectified current is subject to a pulsation due to the voltage waveform of the AC power supply. In many cases, this pulsation is reduced by inserting a filter or a smoothing reactor between the power converter system and the load, thus damping the pulsation of the power supplied to the load.
Notwithstanding, the problem posed by the power converter system of this type in operation is the higher harmonics current produced at the AC power supply side by the commutation of the rectifiers. Essentially, the functions of a rectifier are limited to the conduction and cut-off of current flowing therein. At the time of commutation of the rectifier, therefore, the AC current at the AC power supply side undergoes a sharp change and takes a form similar to a rectangular wave. The AC power supply thus comes to contain higher harmonics components.
The power converter system, when mounted on an electric car, for example, causes such higher harmonics current to have an adverse effect on nearby communication equipment through the trolley wires, resulting in an inductive interference or other troubles.
A method has been so far suggested in which a retardation coil is inserted in the AC circuit connecting the AC power supply and the power converter system, so that the variations of the AC input current at the time of commutation of the rectifiers is dampened by the inductance of the retardation coil to thereby reduce the higher harmonics current.
Even though the retardation coil is effective for reduction of the higher harmonics current, an increased inductance in the AC circuit generally causes a decreased power factor, thereby giving rise to a reduced output of the power converter system. The result is in that it is necessary to increase the size and capacity of not only the power converter system but also the devices on the AC power supply side in order to attain a desired output.
Take a single-phase rectifier circuit as an example of the power converter system. The variations of the AC input current due to the combination of the rectifiers depend on the voltage magnitude of the AC power supply itself as a source of the commutation energy. In the event that the voltage of the AC power supply takes a sinusoidal waveform, the variations of the AC input current are sharp at or near the 90.degree. phase of the AC voltage associated with the commutation, and slow at or near 0.degree. or 180.degree..
Therefore, if the inductance of the commutation circuit making up a passage of the commutation current is smaller at a maximum or minimum output of the power converter system than the inductance when the output is not maximum or minimum, the higher harmonics components of the AC input current may be generally reduced on the one hand and the efficiency of the power converter system can be improved on the other. It is from this point of view that the present invention has been made.